A steam turbine power generating system is designed so that a linearity is maintained as a characteristic in a relation between an opening degree of a steam control valve provided in a main steam pipe which sends steam to a steam turbine and a flow rate of steam passing through the steam control valve.
However, in practice, directly before the opening degree of the steam control valve is made fully opened, it becomes difficult to make a supply amount of steam follow an increased amount of the opening degree of the steam control valve. Such a phenomenon is caused by a fact that an opening size of a flow path inside a pipe connected to a downstream side of the steam control valve becomes comparatively smaller than an opening size of a flow path inside the valve when the steam control valve is being opened.
Thus, a technique is suggested in which a bypass pipe provided with a steam overload valve is disposed, in consideration of an operation at a time of overload of a steam turbine. Such a bypass pipe is connected to a lower-pressure side of the steam turbine than a main steam pipe, while bypassing a steam control valve from on a main steam pipe, for example.
However, supplying steam from a bypass pipe to a low-pressure side of a steam turbine means decrease of a workload performed by steam to the steam turbine for a range from a steam discharge port of a main steam pipe to a steam discharge port of a bypass pipe, compared with a case where steam is supplied from the main steam pipe. Therefore, in order to operate the steam turbine efficiently, it is important to properly control action of a steam overload valve on the bypass pipe.